Processes for the manufacture of acetic acid from methanol by carbonylation are operated extensively throughout the world. A thorough review of these commercial processes and other methods for the production of acetyl compounds from single carbon sources are described by Howard et al. in Catalysis Today, 18 (1993) 325-354. All commercial carbonylation processes for the preparation of acetic acid involve feeding methanol, a halogen compound, typically hydrogen iodide and/or methyl iodide, and a solvent such as acetic acid to a reaction zone wherein the feed materials are contacted with carbon monoxide and a Group VIII catalyst, typically a rhodium catalyst. The liquid reaction mixture is removed from the reaction zone and the product acetic acid and/or other acetyl compound is recovered from the liquid. In the most important carbonylation processes, i.e., the conversion of methanol to acetic acid and the conversion of methyl acetate to acetic anhydride, hydrogen iodide and/or methyl iodide normally are fed to the reaction zone wherein the carbonylation reaction occurs. The feed of hydrogen iodide and/or methyl iodide is problematic since the hydrogen iodide and/or methyl iodide are corrosive, must be removed from the product and recycled in subsequent distillation steps, and due to its toxicity and volatility, requires very rigorous and expensive process controls. Elimination of the requirement to add this large volume of methyl iodide would reduce significantly the costs associated with separation and the expensive control equipment associated with safely handling such a volatile and toxic component.
A review of these processes is available in Howard, et. al., Catalysis Today, 18, 325-354 (1993). Included in the Howard et. al. article is a listing of attempts to develop an alkyl halide-free carbonylation system (see pages 345-347). However, all previous attempts have failed to provide a commercially-viable process since alkyl halide-free carbonylation processes give very slow reaction rates, proceeding at about 1% or less of the rates of the commercial process.